Adsorbent clays are widely used to remove color and odor from vegetable and mineral oils. Two different methods of presenting the clays are utilized: contact filtration and percolation. In the contact filtration method, a finely divided adsorbent is mixed with the solution to be treated, agitated, and removed from the treated solution by settling and filtration. For percolation, the adsorbent is held in place and the liquid stream to be treated is made to flow over, around, and through it. In the percolation method, the adsorbent usually is in granular form and must have certain mechanical and physical requirements in order to hold its shape and be retained in place.
Adsorbent clays are also used to purify water. The contact filtration method of applying adsorbent clay to water selectively precipitates some contaminants in the form of sludges which are subsequently disposed of in landfills. For example, U.S. Pat. No. 4,765,908 to Monick et al. describes a process for removing contaminants from a wastewater system by precipitating them as a non-leachable sludge. The process utilizes a precipitation-inducing composition containing bentonite clay which is said to remove contaminants such as phosphates, nitrites, nitrates, and heavy metals from wastewater.
The process of the '908 patent, and contact filtration processes in general, have drawbacks for treating aqueous effluent in that they contact clay and wastewater in a relatively unconfined manner in an open tank. The resulting sludge must be collected before it can be disposed of. Also before disposal, the sludge must be dewatered using a belt filter, wringer, press roller, or the like. Plant personnel necessarily risk exposure to the contaminants during the collection, dewatering, and disposal operations.
Some specially selected and prepared adsorbent clays are capable of holding their shape while in contact with water and are, therefore, suitable for use with the percolation method. U.S. Pat. No. 4,054,515 to Sawyer, Jr. reports that percolation through a column of 30 to 60 mesh low volatile material (LVM) attapulgite clay particles can remove a toxin, a pesticide, a hormone, and heavy metal ions from various aqueous solutions. The '515 patent goes on to suggest that the clay particles be placed in a downflow metal or plastic container with a filter at the bottom to prevent the clay particles from passing out of the container during operation. The patent states further that other adsorbing materials, such as charcoal and sand, can be placed in the container with the clay particles.
However, there is room for further improvement in the application of adsorbent clays to wastewater processing. The simple particulate beds described in the '515 patent are not suitable for use with aqueous effluent streams that contain suspended solids. For example, the '515 patent provides no means to keep the suspended solids from reaching the particulate bed, teaches no means for immobilizing the suspended solids within the bed, and makes no provision for redistributing liquid flow patterns which have been altered or disturbed by agglomerations of the suspended solids. In addition, the particulate beds of the '515 patent are not adapted for use with relatively large volume, industrial scale waste streams.
U.S. Pat. No. 3,733,267 to Haase refers to a cross flow filter cartridge that incorporates a primary filter surrounding an adsorber unit which contains heat-treated attapulgite clay particles of about 6 to 25 mesh size. The cross flow filter cartridge uses concentric, perforated cylinders to contain the clay particles. An external post-filter is sometimes located downstream of the cartridge. The cartridge is said to remove dissolved foreign materials such as fatty acids, colors, and water from streams that may contain small particles, such as dry cleaning fluid or engine oil. The clay particles may be mixed with particles of adsorbent carbon.
Cross flow cartridges, also called radial flow cartridges, lack the simplicity and flexibility required for industrial wastewater treating. Cross flow filters are relatively complicated and have fragile internal parts that must be carefully fitted. Mechanical limitations usually prevent cross flow filters from being backflushed to dislodge foreign particles. They are inherently susceptible to partial plugging, which in turn results in maldistributed liquid flow and inefficient utilization of the adsorbent. Further, only a fraction of the volume in a cross flow filter cartridge is actually occupied by adsorbent material.
Thus, a system is needed that allows a column of absorbent clay particles to be brought into contact with an industrial scale aqueous effluent stream in a manner that exposes a relatively large volume of clay to the stream, yet protects the clay particle column from plugging by suspended solids that may be present in the effluent. Desirably, such a system includes means for minimizing the amount of suspended solids which travel to the particulate bed as well as means for immobilizing the suspended solid and redistributing the flow of liquid when suspended solids enter the particulate bed. It would also be desirable to have an indicator that signals when backflushing of suspended solids should be performed.
Preferably, such a system provides the above-mentioned advantages while allowing relatively easy inspection and replacement of the adsorbent material.
The present invention satisfies the foregoing desires by providing an effluent treatment system that includes a cartridge that resists partial plugging and maldistribution and which can be backflushed.